The present invention relates to an improved process for the preparation of multinuclear aromatic polyamines by the condensation of aniline with formaldehyde in the presence of water and acid catalysts and working up of the reaction mixture by extraction with a hydrophobic solvent. The acid catalyst obtained in the aqueous phase of extraction is reused.
It is already known that in the preparation of multinuclear aromatic polyamines by the condensation of aniline with formaldehyde in the presence of water and acid catalysts, the aqueous reaction mixture obtained may be worked up by extraction with a hydrophobic solvent and the acid catalyst obtained in the aqueous phase in the process of extraction may be reused (see e.g., U.S. Pat. Nos. 3,996,283; 3,952,042, 4,061,678, 4,093,658, and 4,087,459: and German Offenlegungsschrift No. 2,343,658).
The important advantage of the processes described in these prior publications is that the catalyst need not be neutralized since it is obtained in the aqueous phase when the acid reaction mixture is worked up by extraction and may be returned to the beginning of the process in this form and reused. Furthermore, certain variations of this known principle, such as those described in U.S. Pat. Nos. 4,093,658 and 4,087,459, for example, provide for the controlled production of polyamine mixtures containing a selectively increased or reduced proportion of 2,4'-isomers. The products of the processes of the prior publications are otherwise similar in their suitability as preliminary products for the preparation of polyisocyanates to conventional polyamines of the diphenyl methane series which have been prepared with neutralization of the acid catalyst used. This means that the properties of polyurethane foams prepared from such polyisocyanate mixtures of the diphenyl methane series are qualitatively approximately the same in both cases. The processes of the above mentioned prior publications have the disadvantage that extraction requires very considerable quantities of hydrophobic solvents, based on the quantity of extracted product obtained (the concentration of product of the process in the organic phases is generally below 15% by weight), which of course entails considerable expenditure for distillation and hence a high consumption of energy for working up the organic phases by distillation.
It was an object of the present invention to provide a new, improved process for the preparation of multinuclear aromatic polyamines from aniline and formaldehyde in the presence of acid catalysts which would combine the advantages of the known art processes and would in addition enable qualitatively improved products to be obtained with less expenditure of effort for distillation and hence less energy consumption.